Excimer lasers are currently becoming the workhorse light source for the integrated circuit lithography industry. A typical prior art KrF excimer laser is depicted in FIG. 1 and FIG. 9. A pulse power module AE provides electrical pulses lasting about 100 ns to electrodes 6 located in a discharge chamber 8. The electrodes are about 28 inches long and are spaced apart about 3/5 inch. Typical lithography lasers operated at a high pulse rate of about 1,000 Hz. For this reason it is necessary to circulate a laser gas (about 0.1 percent fluorine, 1.3 percent krypton and the rest neon which functions as a buffer gas) through the space between the electrodes. This is done with tangential blower 10 located in the laser discharge chamber. The laser gasses are cooled with a heat exchanger also located in the chamber. Commercial excimer laser system are typically comprised of several modules which may be replaced quickly without disturbing the rest of the system. Principal modules are shown in FIG. 1 and include:
Laser Chamber 8, PA1 Pulse Power Module 2, PA1 Output coupler 16, PA1 Line Narrowing Module 18 PA1 Wavemeter 20 PA1 Computer Control Unit 22 PA1 Peripheral Support Sub systems PA1 Blower 10
The discharge chamber is operated at a pressure of about three atmospheres. These lasers operate in a pulse mode at about 600 Hz to about 1,000 Hz, the energy per pulse being about 10 mJ and the duration of the laser pulses is about 15 ns. Thus, the average power of the laser beam is about 6 to 10 Watts and the average power of the pulses is in the range of about 700 KW. A typical mode of operation is referred to as the "burst mode" of operation. In this mode, the laser produces "bursts" of about 50 to 150 pulses at the rate of 1,000 pulses per second. Thus, the duration of the burst is about 50 to 150 milliseconds. Prior art lithograph excimer lasers are equipped with a feedback voltage control circuit which measures output pulse energy and automatically adjusts to the discharge voltage to maintain a desired (usually constant) output pulse energy. It is very important that the output pulse energy be accurately controlled to the desired level.
It is well known that at wavelengths below 300 nm there is only one suitable optical material available for building the stepper lens used for chip lithography. This material is fused silica. An all fused silica stepper lens will have no chromatic correction capability. The KrF excimer laser has a natural bandwidth of approximately 300 pm (full width half maximum). For a refractive system (with NA&gt;0.5)--either a stepper or a scanner--this bandwidth has to be reduced to below 1 pm. Current prior art commercially available laser systems can provide KrF laser beams at a nominal wave length of about 248 nm with a bandwidth of about 0.8 pm (0.0008 nm). Wavelength stability on the best commercial lasers is about 0.25 pm. With these parameters stepper makers can provide stepper equipment to provide integrated circuit resolutions of about 0.3 microns. To improve resolution a more narrow bandwidth is required. For example, a reduction of a bandwidth to below 0.6 pm would permit improvement of the resolution to below 0.25 microns.
It is known that the addition of very small quantities of oxygen to an excimer laser gas mixture can be used to stabilize the efficiency and performance of the laser. See, for example, U.S. Pat. No. 5,307,364.
The actual performance of integrated circuit lithography equipment then depends critically on maintaining minimum bandwidth of the laser throughout it's operational lifetime.
Therefore, a need exists for a reliable, production quality excimer laser system, capable of long-term factory operation and having accurately controlled pulse energy, wavelength, and a bandwidth.